The present invention relates to the field of hydrostatic units such as transmissions, pumps and motors. More particularly, this invention relates to closed cavity or reduced oil volume pistons that slidably reciprocate in the cylinder bores of hydrostatic units.
Conventional closed cavity pistons have been utilized in hydrostatic units for a variety of agricultural, turf care and construction equipment. One type of conventional closed cavity or reduced oil volume piston has an elongated cylindrical main body and a truncated cylindrical cap. One end of the main body is closed and the other end has a deep target-shaped or annular cavity formed therein by a relatively expensive xe2x80x9ctarget drillingxe2x80x9d operation. The target drilling operation leaves a centrally located stem that is integral with the closed end of the main body and protrudes to the open end. The cap is cold formed and has opposite closed and open ends. The open end includes an annular groove and truncated stem portion such that the cap registers and mates with the stem and outer wall of the main body at its open end. This conventional piston is assembled by adding a washer between the stem and inner wall of the main body near the open end. The washer is included to help center and stabilize the stem during welding. Then the cap and main body are inertia-friction welded together along a single transverse plane where their respective open ends meet. The result is a lightweight hollow closed cavity piston, but the cost and complexity of the target drilling operation make it a relatively expensive piston. Considering that hydrostatic units typically require several pistons for each unit, piston cost profoundly affects the overall cost of the units and hydrostatic transmissions they go into. Therefore, there is a need for a closed cavity piston and method of making the same that eliminates the target drilling operation and reduces cost.
The present invention relates to a closed cavity piston for hydrostatic units and methods of making the same. The closed cavity piston of this invention includes an elongated piston body and a separately formed piston cap having an elongated stem and head thereon. The body includes a closed end and an open end with a cavity having a bottom wall adjacent the closed end and an outer wall terminating in a rim at the open end. The stem of the piston cap is friction inertia welded to the bottom wall of the piston body and the head of the piston cap welds to the rim of the piston body so as cover the entrance opening and sealingly enclose the interior cavity. The piston components can both be formed by conventional and relatively inexpensive cold forming techniques. The stem of the cap is inserted into the cavity of the body until it engages the bottom wall. Then the cap is friction inertia welded to the body at the stem/bottom wall and head/rim interfaces after preheating.